The catalytic vapor-phase hydrogenation of carboxylic acids and esters to form the corresponding aldehydes is well known. Hydrogenation processes wherein the carboxylic acids and esters are free of hydrogen on the alpha-carbon, such as benzoic acid and methyl benzoate, are taught in U.S. Pat. No. 4,328,373, issued on May 4, 1982 to Dow Chemical Company. The reaction is performed in the presence of metal oxide catalysts such as oxides of yttrium, zirconium, cerium, praseodymium, thorium and uranium supported on alpha-alumina. The first two oxides are most preferred. This work is also described in King et al., "An In Situ Study of Methyl Benzoate and Benzoic Acid Reduction on Yttrium Oxide by Infrared Spectroscopic Flow Reactor," Journal of Catalysis 76, 274-284 (1982).
U.S. Pat. No. 4,093,661, issued June 6, 1978 to Union Carbide Corporation, shows the vapor phase disproportionation of lower alkanoate esters of alcohols in the vapor phase to produce aldehydes and ketones over metal oxide catalysts. The preferred catalysts include nickel oxide, zinc oxide, and chromium oxide. The use of other metal oxides, namely, oxides of copper, titanium, vanadium, manganese, iron and cobalt, are also disclosed. It is further taught that the catalyst can be supported on an inert catalyst support such as alumina, silica and carbon.
In U.S. Pat. No. 2,018,350, issued on Oct. 22, 1935 to General Aniline Works, Inc., aldehydes are formed from dicarboxylic acids or their anhydrides with reducing gas in the presence of catalysts such as chromium, iron, copper, manganese, cobalt or their oxides, either alone or mixed with each other. Mixtures of such catalysts with other elements such as lead, cerium, uranium or zinc or oxides of these elements are also generally disclosed. The catalysts include granules of pumice impregnated with iron salts and reduced with hydrogen to prepare benzaldehyde from phthalic anhydride. In addition, a catalyst prepared by reducing fragments of ferric oxide activated with chromium compounds is also described. Other catalysts include copper and iron deposited on granular pumice, a reduced mixture of lead oxide, chromium oxide, and iron oxide, and reduced zinc chromate.